The present invention relates to digital communication systems, and, in particular, to forward error correction through trellis coded modulation.
In recent years, much of the research and development in the communications industry has been concentrated in the area of digital signal transmission. As is well known in the art, digital signal transmission typically involves transmission of data with a carrier frequency. The carrier frequency is modulated by data so that a frequency bandwidth is occupied by the transmitted signal. The growing demand for access to data and communication services has placed a significant strain on the available bandwidth. Moreover, there is an ever increasing demand for increased data communication rates for the purpose of decreasing the data transmission time. An increase of the rate of the data typically results in an increased bandwidth requirement, placing a further strain upon the available bandwidth for transmission of signals.
In an effort to increase the data rates without sacrificing the available bandwidth, a number of increasingly sophisticated coded modulation schemes have been developed. For example, quadrature amplitude modulation (QAM) employs both amplitude and phase modulation in order to encode more data within a given frequency bandwidth. Another modulation technique involves multiple phase shift keying (MPSK) to increase data capacity within a given bandwidth. These high level modulation schemes are very sensitive to channel impairments. That is, the information encoded by means of such techniques is often lost during transmission due to noise, Rayleigh fading and other factors which are introduced over the communication medium.
In order to compensate for the increased sensitivity of these high level modulation schemes, various forward error correction coding techniques are employed. One such error coding technique is trellis coded modulation. Trellis coded modulation is desirable since it combines modulation and error coding operations to provide effective error control coding without sacrificing power and bandwidth efficiency. Furthermore, it has been shown that trellis coded modulation schemes perform significantly better than their uncoded equivalents with the same power and bandwidth efficiency. Trellis codes have been developed for many of the high-level, high-rate modulation schemes, including well-known 8-PSK modulation and Square 16 QAM modulation. However, designers of past systems have not considered providing a technique of trellis coding which applies to any phase and/or amplitude modulation scheme, as well as codes having various constraint lengths, while providing optimal or near optimal error performance.
Typically, a new set of xe2x80x9coptimalxe2x80x9d trellis codes must be found individually for each modulation scheme. The xe2x80x9coptimalxe2x80x9d trellis codes are typically found through algorithms that search all the possible trellis code structures that have simple feedback of feed-forward shift register implementations. Even a small change in system parameters, such as code constraint length, requires a search for an entirely new set of trellis codes.
The present invention provides a system and method for trellis-coded modulation and demodulation of phase and/or amplitude modulated signals complying with varying signal constellations and constraint lengths, while yielding optimum or near optimum error performance. The cyclic trellis encoding method of the present invention can generate a family of trellis codes whose performance is better than or equal to so called xe2x80x9coptimalxe2x80x9d codes generated by other techniques. Generally, the cyclic trellis codes do not have a feed-forward or feed-back shift register implementation.
A method of cyclic trellis encoding a data sequence within an encoder is disclosed. The data sequence is to be mapped according to a predetermined modulation scheme having an associated signal constellation. The signal constellation has defined coordinate points corresponding to phase and amplitude characteristics corresponding to output symbols from the encoder. The method comprises the step of defining an output table of output symbols. The output table has present state rows and input symbol columns. The output symbols are determined as a function of symbols input to the encoder and a present state of the encoder. The method of the present invention further comprises the step of defining the output table, which further comprises the substeps of assigning each of the output symbols to the points of the signal constellation; partitioning the points of the signal constellation into a first subset of output symbols and a second subset of output symbols; loading even ones of the present state rows with output symbols from the first subset; and loading odd ones of the present state rows with output symbols from the second subset. The method further comprises the step of defining a next-state table of next states for the encoder. The next-state table has present state rows and input symbol columns, wherein the next states are defined as a function of symbols input to the encoder and a present state of the encoder. The method further comprises the step of defining the next-state look-up table, which further comprises the substeps of loading first ones of the present state rows with next states of the encoder until at least one of the first present state rows is full and all of the next state values have been used; and loading other ones of the present state rows with next states that are cyclicly shifted from the next states in each of the first ones of the present state rows until all of the present state rows are filled. The method of the present invention further comprises the step of implementing the output and next-state tables within the encoder so that output symbols from the encoder are determined by input symbols to the encoder and the present state of the encoder in accordance with the output table, and transitions from the present state of the encoder to the next state of the encoder are performed in accordance with the next-state table. Finally, the method comprises the step of mapping the output symbols into signals having phase and amplitude characteristics corresponding to points on the signal constellation.
In a preferred embodiment of the present invention, the coordinate points of the signal constellation are assigned output symbols according to natural mapping techniques when the predominant channel interference is Additive White Gaussian Noise.
In another preferred embodiment the coordinate points of the signal constellation are assigned to output symbols according to Gray coding techniques when the predominant channel interference is Rayleigh fading.
In another embodiment of the present invention, a method of cyclic trellis encoding an input data sequence with an encoder is disclosed. The input data sequence is to be mapped according to a predetermined modulation scheme having an associated signal constellation. The signal constellation has defined coordinate points corresponding to phase and amplitude characteristics of output symbols from the encoder. The encoder receives n inputs, corresponding to 2n input values, and outputs n+1 outputs corresponding to 2n+1 output values. The encoder has 2k possible states. The method of the present invention comprises the steps of defining an output table having 2k present state rows and 2n input symbol columns, wherein the output symbols from the encoder are determined as a function of input symbols to the encoder and a present state of the encoder; defining the output table which further comprises the substeps of assigning values to the points of the signal constellation, where the values correspond to the output symbols; partitioning the signal constellation into a first subset of 2n output symbols and a second subset of 2n output symbols, where the first subset and second subset are symmetric; loading even ones of the present state rows with values corresponding to output symbols from the first subset; and loading odd ones of the present state rows with values corresponding to output symbols from the second subset. The method of the present invention further comprises the step of defining a next-state table of a plurality of next states, where the next-state table has 2k present state rows and 2n input symbol columns. The next state of the encoder is determined as a function of the input symbols to the encoder and the present state of the encoder. The step of defining the next-state table further comprises the substeps of dividing the next states into 2kxe2x88x92n subsets wherein each subset has 2n next states; and loading a first one of the present state rows with next states from a first one of the subsets, a second one of the present state rows with the next states from a second one of the subsets, and continuing this loading until the 2kxe2x88x92nth present state row is loaded with the next states from the 2kxe2x88x92nth one of the subsets. The method of the present invention further comprises the steps of implementing the output and next-state tables within the encoder so that output symbols from the encoder are determined as a function of input symbols to the encoder and the present state of the encoder in accordance with the output table, and transitions from the present state of the encoder to the next state of the encoder are in accordance with the next-state table; and mapping the output symbols from the encoder into signals having phase and amplitude characteristics corresponding to respective output symbol points on the signal constellation.
In a preferred embodiment of the invention, the coordinate points of the signal constellation are assigned output symbols according to natural mapping techniques when the predominant channel interference is Additive White Gaussian Noise.
In another preferred embodiment of the invention, the coordinate points of the signal constellation are assigned output symbols according to Gray coding techniques when the predominant channel interference is Rayleigh fading.
Under another aspect, the present invention provides for a trellis encoder which trellis encodes input data signals, wherein the input data signals are mapped according to a modulation scheme such that a signal constellation defined by the modulation scheme cannot be set partitioned such that each level of set partitioning results in a substantially increased minimum Euclidean distance between points of the signal constellation.
Another embodiment of the present invention calls for a transmitter for a trellis-coded, multi-level modulation communication system. The transmitter comprises a cyclic trellis encoder which receives a sequence of data input symbols and outputs a sequence of encoded output symbols. The cyclic trellis encoder has a set of present states partitioned into subsets. The encoder comprises a state transition table containing a plurality of next state values for the encoder. The next state values are defined based upon the present state of the encoder and the input symbol. The next state values are assigned to each of the present state subsets such that the next state values for any present state subset are shifted cyclicly for successive members of the any present state subset. A present state memory element connects to the state transition look-up table and temporarily stores a next state value output by the state transition look-up table. An encoder output look-up table connects to the present state memory element which selects an output symbol based upon the present state of the encoder and the presently received one of the input symbols. The present states and the output values are partitioned into two subsets so that the output look-up table outputs a symbol which belongs to a first output subset when in one of the present state subsets and outputs a symbol which belongs to a second output subset when in the other of the present state subsets. A signal mapper connects to the output look-up table, and maps outputs of the encoder output look-up table into encoded output signals from two symmetric signal constellations. Finally, a transmitter circuit transmits the encoded output signals over a communications medium.
In a preferred embodiment, the signal mapper maps according to Gray coding techniques. In another preferred embodiment, the signal mapper maps according to natural mapping techniques.
Under another aspect, the apparatus of the present invention comprises a receiver for a trellis-coded multi-level modulation communication system. The receiver comprises a trellis decoder which receives a baseband signal. The trellis decoder comprises a means for reconstructing a trellis structure defined by a state transition look-up table wherein a set of present states is partitioned into subsets having successive members, and next state values assigned to each of the present state subsets are shifted cyclicly for successive members of the present state subset. The trellis decoder further comprises a means for determining input and output symbols associated with branches of the trellis structure as defined by an encoder output look-up table. The present states and the output values are partitioned into two subsets so that the output look-up table outputs a symbol which belongs to a first output subset when in one of the present state subsets, and outputs a symbol which belongs to a second output subset when in the other of the present state subsets. The decoder also includes a calculation circuit which determines the Euclidean distances between points on a phase/amplitude coordinate system corresponding to the received signals and points of a phase/amplitude signal constellation corresponding to signals associated with branches on the trellis structure. Finally, the trellis decoder comprises a comparator circuit which selects the most likely path of the received signal on the trellis structure on the basis of the determined Euclidean distances.
In a further embodiment of the present invention, a method is disclosed of forward error correction coding for a data signal mapped according to a given signal constellation. The method comprises the steps of defining a family of convolutional codes. The family of convolutional codes is characterized in that the family of codes are not capable of being generated by means of a feed-forward or feed-back shift register implementation. The step of defining further comprises the substeps of establishing a next-state value corresponding to each present-state/input-value pair; establishing an output value corresponding to each present-state/input-value pair; receiving an input data symbol corresponding to an input value; providing an output value in response to the reception of the input value, wherein the output value is determined by the input value and the present-state value, and the present state value is determined by the previous next state value; generating an output data symbol, wherein the output symbol is determined by the output value; and encoding a data signal to correspond to the output symbol as determined by a signal mapping scheme.
In accordance with a still further aspect of the present invention, a method of encoding signals mapped according to any signal constellation format comprises the steps of dividing the signal constellation points into two symmetrical sets of symbol points, and cyclic trellis encoding data to be mapped according to the signal constellation.
Under a further aspect, the present invention comprises a data encoder. the data encoder comprises a cyclic trellis encoder and a transmitter coupled to the cyclic trellis encoder.
Under a yet further aspect, the present invention comprises a trellis encoder that may be implemented as a state machine or a look-up table memory or in software, but which cannot be implemented as a shift register. In one embodiment, the cyclic trellis encoder is such as not to allow a feed-forward shift register implementation. According to a further aspect, the cyclic trellis encoder is such as not to allow a feed-back shift register implementation.
In yet a further embodiment, the present invention is a data encoder for use in data communication application, wherein a plurality of input data values are trellis encoded to form output data. The data encoder comprises an input and an encoder circuit coupled to the input. The encoder circuit has a plurality of present states, and is responsive to the plurality of input data values on the input to transition to a next state. The next states correspond to each input which is cyclicly shifted for different ones of the present states. The encoder further comprises an output coupled to the encoder circuit. The output is responsive to the encoder circuit to generate output data.
A trellis encoder constructed in accordance with the teachings of the present invention can be adapted to trellis encode data to be mapped for signal constellations. The signal constellations can be set partitioned such that each level of set partitioning results in substantially increased minimum Euclidean distance between points of the signal constellation. The trellis encoder can also be adapted to trellis encode data to be mapped for signal constellations which cannot be set partitioned such that each level of set partitioning results in substantially increased minimum Euclidean distance between points of the signal constellation.